Fw: Intermittent SCTP multihoming breakage

Fw: Intermittent SCTP multihoming breakage

[Andrew Morton]

Begin forwarded message:

Date: Wed, 3 Jan 2007 11:54:26 +0000
From: Steve Hill <steve.hill@dialogic.com>
To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Intermittent SCTP multihoming breakage



Apologies if I'm posting to the wrong list - the lksctp lists seem to be a
bit dead these days and a bit of Googling seemed to inidicate that SCTP
developemnt discussions might have moved here.

I'm running under the 2.6.16.1 kernel and have an intermittent problem
with the SCTP stack.  Having reviewed the git logs I can't see any
indication that the problem has been fixed in more recent kernels, but it
is very difficult to test since it is so intermittent.

I am running a multihomed connection between 2 machines, (2 NICs on
each machine, so 2 paths for the connection) and tcpdump shows heartbeat
requests and acks on both paths.  Putting data over the link correctly
sends it over the first path.

If I drop the traffic on one of the NICs then most of the time it
correctly fails over the the second path and I see the data being sent
and acknowledged correctly on the second path.  However, I also
intermittently see two failure conditions:

1. Sometimes, just after failing over to the second path I see an ABORT.
2. More frequently, the association stays up indefinately, with heartbeat
requests and acks on the second path, but no data chunks are sent even
though the transmit queue on the transmitting end appears to be full and
the socket is blocking writes.

I have been adding debugging to the kernel in an attempt to track down the
source of the second failure condition, and I am wondering if anyone else
has seen similar behaviour?

-- 
 - Steve Hill
   Software Engineer
   Dialogic
   Fordingbridge, Hampshire, UK
   +44-1425-651392
   steve.hill@dialogic.com
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Re: Fw: Intermittent SCTP multihoming breakage

[Sridhar Samudrala]

On Wed, 2007-01-03 at 15:46 -0800, Andrew Morton wrote:
> 
> Begin forwarded message:
> 
> Date: Wed, 3 Jan 2007 11:54:26 +0000
> From: Steve Hill <steve.hill@dialogic.com>
> To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
> Subject: Intermittent SCTP multihoming breakage
> 
> 
> 
> Apologies if I'm posting to the wrong list - the lksctp lists seem to be a
> bit dead these days and a bit of Googling seemed to inidicate that SCTP
> developemnt discussions might have moved here.

No. lksctp-developers mailing list is still the best place for SCTP related
discussions. You can subscribe and look in the archives at 
  http://lists.sourceforge.net/lists/listinfo/lksctp-developers

> 
> I'm running under the 2.6.16.1 kernel and have an intermittent problem
> with the SCTP stack.  Having reviewed the git logs I can't see any
> indication that the problem has been fixed in more recent kernels, but it
> is very difficult to test since it is so intermittent.

If possible, i would suggest moving to the latest mainline 2.6.19.
But 2.6.16.1 should work OK for simple multihoming cases.

> 
> I am running a multihomed connection between 2 machines, (2 NICs on
> each machine, so 2 paths for the connection) and tcpdump shows heartbeat
> requests and acks on both paths.  Putting data over the link correctly
> sends it over the first path.
How are the 2 machines connected? Are they connected directly or
via a router?

Do you see both the addresses when you do cat /proc/net/sctp/assocs 
after the association is established on both the peers?

> 
> If I drop the traffic on one of the NICs then most of the time it
> correctly fails over the the second path and I see the data being sent
> and acknowledged correctly on the second path.  However, I also
> intermittently see two failure conditions:

How are you dropping traffic? You could try simulating failover by
bringing down the interface or physically removing the link.

> 
> 1. Sometimes, just after failing over to the second path I see an ABORT.
This seems to indicate that somehow the app has terminated.

> 2. More frequently, the association stays up indefinately, with heartbeat
> requests and acks on the second path, but no data chunks are sent even
> though the transmit queue on the transmitting end appears to be full and
> the socket is blocking writes.
This is strange. Can you collect tcpdump traces on sender and receiver when 
this happens?

Thanks
Sridhar

> 
> I have been adding debugging to the kernel in an attempt to track down the
> source of the second failure condition, and I am wondering if anyone else
> has seen similar behaviour?
> 
> --
>  - Steve Hill
>    Software Engineer
>    Dialogic
>    Fordingbridge, Hampshire, UK
>    +44-1425-651392
>    steve.hill@dialogic.com
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

Re: Fw: Intermittent SCTP multihoming breakage

[Steve Hill]

On Wed, 3 Jan 2007, Sridhar Samudrala wrote:

Sorry for the delay in replying.

> No. lksctp-developers mailing list is still the best place for SCTP related
> discussions. You can subscribe and look in the archives at
>   http://lists.sourceforge.net/lists/listinfo/lksctp-developers

Hmm, I had a look there and it seemed reasonably inactive and overrun by
spam.. (And I've been unable to subscribe).

> How are the 2 machines connected? Are they connected directly or
> via a router?

They are currently connected together directly through crossover cables.

> Do you see both the addresses when you do cat /proc/net/sctp/assocs
> after the association is established on both the peers?

Yes, the contents of /proc/net/sctp/assocs looks correct.

> How are you dropping traffic? You could try simulating failover by
> bringing down the interface or physically removing the link.

I have been using iptables to drop SCTP packets on both the INPUT and
OUTPUT chains.  However, I get the same results if I just unplug the
network cable (using iptables is easier for my testing since I don't have
to crawl around behind the test systems :)

> > 1. Sometimes, just after failing over to the second path I see an ABORT.
> This seems to indicate that somehow the app has terminated.

The abort _appears_ to be caused by a retransmit timer expiring, causing
the SCTP stack to tear down the association.  However, I haven't done much
investigation of this problem yet - I've been focussing on the second
problem since it seems to happen more frequently.

> > 2. More frequently, the association stays up indefinately, with heartbeat
> > requests and acks on the second path, but no data chunks are sent even
> > though the transmit queue on the transmitting end appears to be full and
> > the socket is blocking writes.
> This is strange. Can you collect tcpdump traces on sender and receiver when
> this happens?

I've taken dumps of the data on the wire for both paths:
  http://www.nexusuk.org/~steve/sctp/path1.pcap
  http://www.nexusuk.org/~steve/sctp/path2.pcap

I can't see anything odd in the network traffic - it just stops as if it
has no more data to send.  However, the socket appears to still be
blocking so the application cannot give it any new data.

This seems to be a problem with the abandonment functionality:
1. Transmit chunk 1.  The transmitted list now contains chunk 1.
2. Chunk 1 and it's retransmissions get lost on the network.
3. Abandon chunk 1.  The transmitted list is now empty.
4. Transmit chunk 2.  the transmitted list now contains chunk 2
5. Receive a gap-ack for chunk 2, indicating that chunk 1 is missing.
At this point, the T3 timer is disabled at the bottom of
sctp_check_transmitted() since all the chunks in the transmitted queue are
gap-acked.  The whole connection now stalls, waiting for the SACK for
chunk 1 that will never arrive.

It should be noted that this is not unordered data and I'm not clear on
how abandoned chunks are supposed to be handled - I hadn't intentionally
enabled the abandonment functionality, the timetolive was set on the
transmitted chunks by accident.

-- 
 - Steve Hill
   Software Engineer
   Dialogic
   Fordingbridge, Hampshire, UK
   +44-1425-651392
   steve.hill@dialogic.com

Re: Fw: Intermittent SCTP multihoming breakage

[Sridhar Samudrala]

On Wed, 2007-01-10 at 11:55 +0000, Steve Hill wrote:
> On Wed, 3 Jan 2007, Sridhar Samudrala wrote:
> 
> Sorry for the delay in replying.
> 
> > No. lksctp-developers mailing list is still the best place for SCTP related
> > discussions. You can subscribe and look in the archives at
> >   http://lists.sourceforge.net/lists/listinfo/lksctp-developers
> 
> Hmm, I had a look there and it seemed reasonably inactive and overrun by
> spam.. (And I've been unable to subscribe).

It is not very active all the time, but you can see bursts of activity. 
All the SCTP patches and issues get discussed there before being 
submitted to netdev.
I didn't realize that so much spam is getting on this list. I don't see
any of it as my mailer's spam filters seem to be pretty effective. I will
see if there is a way to set up spam filters on sourceforge lists.

I haven't heard of any issues with subscription. Could you try again and
let me know offline if you still have issues.

> 
> > How are the 2 machines connected? Are they connected directly or
> > via a router?
> 
> They are currently connected together directly through crossover cables.
> 
> > Do you see both the addresses when you do cat /proc/net/sctp/assocs
> > after the association is established on both the peers?
> 
> Yes, the contents of /proc/net/sctp/assocs looks correct.
> 
> > How are you dropping traffic? You could try simulating failover by
> > bringing down the interface or physically removing the link.
> 
> I have been using iptables to drop SCTP packets on both the INPUT and
> OUTPUT chains.  However, I get the same results if I just unplug the
> network cable (using iptables is easier for my testing since I don't have
> to crawl around behind the test systems :)
> 
> > > 1. Sometimes, just after failing over to the second path I see an ABORT.
> > This seems to indicate that somehow the app has terminated.
> 
> The abort _appears_ to be caused by a retransmit timer expiring, causing
> the SCTP stack to tear down the association.  However, I haven't done much
> investigation of this problem yet - I've been focussing on the second
> problem since it seems to happen more frequently.
> 
> > > 2. More frequently, the association stays up indefinately, with heartbeat
> > > requests and acks on the second path, but no data chunks are sent even
> > > though the transmit queue on the transmitting end appears to be full and
> > > the socket is blocking writes.
> > This is strange. Can you collect tcpdump traces on sender and receiver when
> > this happens?
> 
> I've taken dumps of the data on the wire for both paths:
>   http://www.nexusuk.org/~steve/sctp/path1.pcap
>   http://www.nexusuk.org/~steve/sctp/path2.pcap
> 
> I can't see anything odd in the network traffic - it just stops as if it
> has no more data to send.  However, the socket appears to still be
> blocking so the application cannot give it any new data.
> 
> This seems to be a problem with the abandonment functionality:
> 1. Transmit chunk 1.  The transmitted list now contains chunk 1.
> 2. Chunk 1 and it's retransmissions get lost on the network.
> 3. Abandon chunk 1.  The transmitted list is now empty.
> 4. Transmit chunk 2.  the transmitted list now contains chunk 2
> 5. Receive a gap-ack for chunk 2, indicating that chunk 1 is missing.
> At this point, the T3 timer is disabled at the bottom of
> sctp_check_transmitted() since all the chunks in the transmitted queue are
> gap-acked.  The whole connection now stalls, waiting for the SACK for
> chunk 1 that will never arrive.
> 
> It should be noted that this is not unordered data and I'm not clear on
> how abandoned chunks are supposed to be handled - I hadn't intentionally
> enabled the abandonment functionality, the timetolive was set on the
> transmitted chunks by accident.

So looks like there may be an issue with PR-SCTP(partial reliability)
support and packet loss. I will take a look into this.

Do you still see this problem even if you don't set timetolive?
If you don't need chunks to be expired, you can disable pr-sctp using 
echo 0 > /proc/sys/net/sctp/prsctp-enable

Thanks
Sridhar

Re: Fw: Intermittent SCTP multihoming breakage

[Vlad Yasevich]

Steve Hill wrote:
> On Wed, 3 Jan 2007, Sridhar Samudrala wrote:
> 
> Sorry for the delay in replying.
> 
>> No. lksctp-developers mailing list is still the best place for SCTP related
>> discussions. You can subscribe and look in the archives at
>>   http://lists.sourceforge.net/lists/listinfo/lksctp-developers
> 
> Hmm, I had a look there and it seemed reasonably inactive and overrun by
> spam.. (And I've been unable to subscribe).
> 
>> How are the 2 machines connected? Are they connected directly or
>> via a router?
> 
> They are currently connected together directly through crossover cables.
> 
>> Do you see both the addresses when you do cat /proc/net/sctp/assocs
>> after the association is established on both the peers?
> 
> Yes, the contents of /proc/net/sctp/assocs looks correct.
> 
>> How are you dropping traffic? You could try simulating failover by
>> bringing down the interface or physically removing the link.
> 
> I have been using iptables to drop SCTP packets on both the INPUT and
> OUTPUT chains.  However, I get the same results if I just unplug the
> network cable (using iptables is easier for my testing since I don't have
> to crawl around behind the test systems :)
> 
>>> 1. Sometimes, just after failing over to the second path I see an ABORT.
>> This seems to indicate that somehow the app has terminated.
> 
> The abort _appears_ to be caused by a retransmit timer expiring, causing
> the SCTP stack to tear down the association.  However, I haven't done much
> investigation of this problem yet - I've been focussing on the second
> problem since it seems to happen more frequently.
> 
>>> 2. More frequently, the association stays up indefinately, with heartbeat
>>> requests and acks on the second path, but no data chunks are sent even
>>> though the transmit queue on the transmitting end appears to be full and
>>> the socket is blocking writes.
>> This is strange. Can you collect tcpdump traces on sender and receiver when
>> this happens?
> 
> I've taken dumps of the data on the wire for both paths:
>   http://www.nexusuk.org/~steve/sctp/path1.pcap
>   http://www.nexusuk.org/~steve/sctp/path2.pcap

Taking a look at these it does appear to complete stall...  There are some
rather interesting retransmission that don't look quite right...

> 
> I can't see anything odd in the network traffic - it just stops as if it
> has no more data to send.  However, the socket appears to still be
> blocking so the application cannot give it any new data.
> 
> This seems to be a problem with the abandonment functionality:
> 1. Transmit chunk 1.  The transmitted list now contains chunk 1.
> 2. Chunk 1 and it's retransmissions get lost on the network.
> 3. Abandon chunk 1.  The transmitted list is now empty.

This causes a FORWARD TSN chunk to be sent to the peer telling him
to advance CTSN to that of chunk 1.

> 4. Transmit chunk 2.  the transmitted list now contains chunk 2
> 5. Receive a gap-ack for chunk 2, indicating that chunk 1 is missing.

Yes, but at this point, we will regenerate the FORWARD TSN since chunk1
is still on the abandoned list.

> At this point, the T3 timer is disabled at the bottom of
> sctp_check_transmitted() since all the chunks in the transmitted queue are
> gap-acked.  The whole connection now stalls, waiting for the SACK for
> chunk 1 that will never arrive.
> 

I'll look some more at this...

-vlad
> It should be noted that this is not unordered data and I'm not clear on
> how abandoned chunks are supposed to be handled - I hadn't intentionally
> enabled the abandonment functionality, the timetolive was set on the
> transmitted chunks by accident.
> 

Re: Fw: Intermittent SCTP multihoming breakage

[Steve Hill]

On Wed, 10 Jan 2007, Sridhar Samudrala wrote:

> So looks like there may be an issue with PR-SCTP(partial reliability)
> support and packet loss. I will take a look into this.
>
> Do you still see this problem even if you don't set timetolive?

No, the problem seems to go away if the timetolive is set to 0, so this is
what I have now done since I had not intended to set the timetolive in the
first place (but I thought it was still worth posting details of the
problem since it does appear to be a bug).

-- 
 - Steve Hill
   Software Engineer
   Dialogic
   Fordingbridge, Hampshire, UK
   +44-1425-651392
   steve.hill@dialogic.com

Re: [Lksctp-developers] Fw: Intermittent SCTP multihoming breakage

[Vlad Yasevich]

[-- Attachment #1: Type: text/plain, Size: 1190 bytes --]

Hi Steve

Steve Hill wrote:
> On Wed, 10 Jan 2007, Sridhar Samudrala wrote:
> 
>> So looks like there may be an issue with PR-SCTP(partial reliability)
>> support and packet loss. I will take a look into this.
>>
>> Do you still see this problem even if you don't set timetolive?
> 
> No, the problem seems to go away if the timetolive is set to 0, so this is
> what I have now done since I had not intended to set the timetolive in the
> first place (but I thought it was still worth posting details of the
> problem since it does appear to be a bug).
> 

I think I found this bug.  It was rather interesting to figure out.  The problem
appears to be that data messages time-out within the rto.  As a result, they
move the abandoned list and are never retransmitted.  This clears the retransmit
list and the retransmit timer, however the data is still charged as in-flight against
the association.  This in turn causes new data not to be send, since we are 'supposedly'
utilizing our congestion window.

Can you try the attached patch and let me know if the problem is fixed.  You can 
try reducing rto_max or path_max_retrans to get the failover to happen a little faster.

Regards
-vlad

[-- Attachment #2: 0001-SCTP-Fix-connection-hang-slowdown-with-PR-SCTP.txt --]
[-- Type: text/plain, Size: 2804 bytes --]

[SCTP]: Fix connection hang with PR-SCTP

The problem that this patch corrects happens when all
of the following conditions are satisfisfied:
    1.  PR-SCTP is used and the timeout on the chunks is
        set below RTO.Max.
    2.  One of the paths on a multihomed associations
        is brought down.

In this scenario, data will expire within the rto of the
initial transmission and will never be retransmitted.  However
this data still fills the send buffer and is counted against
the association as outstanding data.  This causes any new
data to not be sent and retransmission to not happen.

The fix is to discount the abandoned data from the outstanding
count and peers rwnd estimation.  This allows new data to
be sent and a retransmission timer restarted.  Even though
this new data will most like expire withing the rto, the
timer still counts as a strike agains the transport and forces
the FORWARD-TSN chunk to be retransmitted as well.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
---
 net/sctp/outqueue.c |   27 ++++++++++++++++++++++-----
 1 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index fba567a..54d1b7f 100644
--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -396,6 +396,19 @@ void sctp_retransmit_mark(struct sctp_outq *q,
 		if (sctp_chunk_abandoned(chunk)) {
 			list_del_init(lchunk);
 			sctp_insert_list(&q->abandoned, lchunk);
+
+			/* If this chunk has not been previousely acked,
+			 * stop considering it 'outstanding'.  Our peer
+			 * will most likely never see it since it will
+			 * not be retransmitted
+			 */
+			if (!chunk->tsn_gap_acked) {
+				chunk->transport->flight_size -=
+						sctp_data_size(chunk);
+				q->outstanding_bytes -= sctp_data_size(chunk);
+				q->asoc->peer.rwnd += (sctp_data_size(chunk) +
+							sizeof(struct sk_buff));
+			}
 			continue;
 		}
 
@@ -1244,6 +1257,15 @@ static void sctp_check_transmitted(struct sctp_outq *q,
 		if (sctp_chunk_abandoned(tchunk)) {
 			/* Move the chunk to abandoned list. */
 			sctp_insert_list(&q->abandoned, lchunk);
+
+			/* If this chunk has not been acked, stop
+			 * considering it as 'outstanding'.
+			 */
+			if (!tchunk->tsn_gap_acked) {
+				tchunk->transport->flight_size -=
+						sctp_data_size(tchunk);
+				q->outstanding_bytes -= sctp_data_size(tchunk);
+			}
 			continue;
 		}
 
@@ -1695,11 +1717,6 @@ static void sctp_generate_fwdtsn(struct sctp_outq *q, __u32 ctsn)
 		 */ 
 		if (TSN_lte(tsn, ctsn)) {
 			list_del_init(lchunk);
-			if (!chunk->tsn_gap_acked) {
-				chunk->transport->flight_size -=
-					sctp_data_size(chunk);
-				q->outstanding_bytes -= sctp_data_size(chunk);
-			}
 			sctp_chunk_free(chunk);
 		} else {
 			if (TSN_lte(tsn, asoc->adv_peer_ack_point+1)) {
-- 
1.4.4.2.g8336

Re: [Lksctp-developers] Fw: Intermittent SCTP multihoming breakage

[Vlad Yasevich]

BTW, if anyone needs a reproducer, I can provide one.

-vlad